Add a small stub which verifies whether /data/data/com.termux/files/usr is readable
into few important packages such like Bash or APT. If this check fails, e.g.
in case if binary was executed in program with different package name (non-Termux),
program will print warning and exit with error status immediately.
This should prevent use of prebuilt packages by third-party applications.
NOTE: for now it uses /data/data/com.termux/files/usr in reversed order to
prevent automatic "blind" patching with sed. Will replace with randomized
XOR-based variant in case if that measure won't be enough.
Leonid Pliushch